Electronic timing apparatus for controlling the duration of light emission of a flash unit

ABSTRACT

An improved arrangement for adjusting the operating duration of a flash tube includes an interruptable thyristor gate in series with the main electrodes of the flash tube for selectively coupling operating voltage from the flash capacitor to the tube. When an ignition pulse is applied to the trigger electrode of the flash tube, a selectable reference voltage derived from the ignition pulse is applied to a Schmitt trigger circuit to generate a voltage of a first polarity. This voltage opens the thyristor gate to energize the main electrodes of the flash tube, thereby causing the tube to emit a flash of light. A portion of the corresponding light reflected from an object illuminated by the flash tube is detected by a photosensitive element and integrated to form a control signal. The control signal is applied to the input of the Schmitt trigger for a duration sufficient to overcome the reference input voltage and thereby reverse the polarity of the output voltage. This latter voltage disables the thyristor gate and terminates the flash.

United States Patent [72] Inventor Rolf Dieter Dennewitz 1,350,60310/1967 Erickson 315/151 Berhn Germany Primary Examiner-Raymond F.Hossfeld [21] Appl' 874935 Att0rneyArthur O. Klein [22] Filed Nov. 7,1969 [45] Patented Oct. 12, 1971 [73] Assignee Loewe Opta G.m.b.H.

Berlin, Germany [32] Prlomy 1969 ABSTRACT: An improved arrangement foradjusting the [33] Germany operating duration of a flash tube includesan interruptable {31] P19 05 223-4 thyristor gate in series with themain electrodes of the flash tube for selectively coupling operatingvoltage from the flash [54] ELECTRONIC TIMING APPARATUS FOR capacitor tothe tubehwhen an ignition pulse is applied to the CONTROLLING THEDURATION OF LIGHT trigger electrode of t e tube, a selectable referencevolt- EMISSION 0F AFLASH UNIT age derived from the ignition pulselsfapphed to a Schmltt 7 Claims, 1 Drawing Fig trigger circuit togenerate a voltage 0 a first polarity. This voltage opens the thyristorgate to energize the mam elec- U.S. trodes of the tube thereby causingthe tube to emit 3 250/205, 31 flash of light. A portion of thecorresponding light reflected [51] Int. Cl ..l-105b 41/32 f an Objectilluminated by the fl h tube is detected by a [50] Field of Search250/205; photosensitive element and integrated to f a m 315/151, 209 R,209 240 signal. The control signal is applied to the input of theSchmitt 56 R f d trigger for a duration sufficient to overcome thereference 1 e erences I e input voltage and thereby reverse the polarityof the output UNITED STATES PATENTS voltage. This latter voltagedisables the thyristor gate and ter- 3,033,988 5/1962 Edgerton 315/151 Xminates the flash.

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ELECTRONIC TIMING APPARATUS FOR CONTROLLING THE DURATION OF LIGHTEMISSION OF A FLASII UNIT BACKGROUND OF THE INVENTION ln recent yearsvarious devices employing light integrating means have been proposed forautomatically adjusting the operating duration of a flash device toobtain a controllable quantity of light energy therefrom. Examples ofsuch devices are described, e.g., in U. S. Pat. No. 3,033,988 issued toH. E. Edgerton; and in U. S. Pat. Nos. 3,350,603 and 3,350,604 issued toR. D. Erickson.

These devices conventionally employ a controllable switch (generally aquench tube of the gas discharge type) which is connected in shunt withthe flash tube and made conductive when the integrated light derivedfrom the emission of the flash tube has reached a predetermined value.Such excitation of the quench tube whose impedance is then far lowerthan that of the flash tube, effects the premature termination of theflash by causing the exciting energy for the tube (e.g. from the flashcapacitor) to bypass the flash tube and be dissipated in the shuntingquench tube.

This arrangement has several disadvantages. Firstly, the dumping" of aportion of the exciting energy into the shunting quench tube representsa loss of efficiency of the apparatus since such shunted energy does notresult in useful light output. Moreover, since the flash capacitor maybe completely discharged through the flash tube and later, through thequenching tube, the flash capacitor must be completely recharged from acompletely discharged or nearly completely discharged state to itsmaximum voltage between successive device and increasing its recyclingtime.

SUMMARY OF THE INVENTION The speed and efficiency of a flash-unitemploying light-integrating, automatic flash duration adjustmentfacilities may be improved with the arrangement of the presentinvention. A normally disabled gate, which can be switched both on andoff by pulses of respectively opposite polarity applied to a controlelectrode thereof, is interposed in the main energizing path of theflash tube for selectively coupling operating voltage (e.g., from theflash capacitor) to the tube when an ignition pulse is applied to itstrigger electrode.

A photosensitive element associated with the flash tube outputcooperates with a capacitor to integrate a portion of the resultinglight energy from the flash tube and to generate a control signal thatvaries in proportion to the amount of light integrated. A Schmitttrigger or other suitable bipolar control device coupled to thephotosensitive element is initially excited with a reference voltagederived from the ignition pulse for opening the gate in the energizingpath of the flash tube to permit the flash capacitor to dischargethrough the tube, whereupon the latter emits a flash of light. When thecontrol signal developed in the integrator in response to such light hasreached a predetermined value sufficient to overcome the referencevoltage at the input of the Schmitt trigger, the output polarity of thetrigger reverses to reclose the gate in the flash tube energizing path,thereby terminating the flash.

Such a flash termination technique is effective to instantaneously stopthe discharge of the flash capacitor so that the latter does not have tobe completely recharged from zero after each flash termination; thus,the recycling time of the unit is improved. Moreover, since only theexact amount of flash capacitor energy needed to produce useful lightoutput from the device is consumed, the arrangement is highly efficient.

In an illustrative embodiment, the gate in the flash unit energizingcircuit is an interruptable thyristor whose transconductive path isinterposed in series with the main electrodes of the flash tube. Thecontrol electrode for this type of thyristor is responsive to signals ofopposite polarities for switching the transconductive path into or outof its conductive state, respectively.

BRIEF DESCRIPTION OF THE DRAWING The nature of the invention will appearmore fully from the following detailed description taken in conjunctionwith the appended drawing, whose single figure shows an illustrativearrangement for automatically adjusting the operating duration of aflash device in accordance with the invention.

DETAILED DESCRIPTION Referring now to the drawing, there is illustrateda conventional flash tube 5 (illustratively a gas filled envelope) whichis provided with a pair of main excitation terminals 21 and 22. The mainterminals are coupled through a normally disabled, controllable gate 6and a resistor 3 to the terminals of a conventional flash capacitor 2.The flash capacitor may be selectively charged in a known manner with ahigh voltage, of the polarity shown, from a conventional DC power sourceI through a diode 23.

The tube 5 is further provided with a trigger electrode 13 which iscoupled to a secondary winding 24 of an ignition transformer 10. Asdescribed hereafter an igniting pulse may be applied to the electrode 13through the transformer 10 by closing a pair of contacts 8 associatedwith a primary winding 9 of the transformer 10.

The voltage of the charged flash capacitor 2, which is coupled to thetube 5 when the gate 6 is open, is not normally sufficient by itself tocause emission of light energy from the tube 5. However, in aconventional manner such tube will emit light upon the concurrentapplication of an igniting pulse to the trigger electrode 13 through theignition transformer 10. The resulting flash of light energy produced inthe tube 5 is emitted through the walls of the envelope toward asuitable object (not shown). A portion of such light is detected byphotosensitive element 14 and integrated by a capacitor 15 in the mannerdescribed below.

As is well known the maximum time duration of the flash of aconventional 'flash tube is essentially determined by the dischargecharacteristic of the flash capacitor 2 through the flash tube. As willbe described presently, the instant invention automatically adjusts theflash duration in a more rapid and efficient manner than heretofore inthose instances where the object to be illuminated requires less thanthe maximum amount of light energy obtainable from the tube in a singleflash.

During the time that the gate 6 remains closed, the stored voltage onthe flash capacitor 2 is coupled to the primary winding 9 of theignition transformer through (I) a voltage divider consisting of a pairof resistors 27 and 28, the latter being adjustable; (2) a pair ofadditional resistors 29 and 31; and (3) a first capacitor 7. As aresult, prior to ignition of the tube 5 the high positive voltage acrossthe flash capacitor 2 is also stored with the polarity indicated acrossthe capacitor 7. The flash capacitor voltage also operates aconventional in dicator lamp 4, which lights up indicating readiness forflash in a known manner. The indicator lamp 4 is coupled as shownbetween the upper terminal of the resistor 27 and a wiper 32 of theadjustable resistor 28.

The gate 6 may be a conventional interruptable" thyristor of the typeused in various power circuits in television practice. Such aninterruptable" thyristor differs from ordinary thyristors of thethyratron type, in which a pulse applied to a control electrode thereof,after initiating conduction in the transconductive path of the device,is thereafter ineffective to return the device to its nonconductivestate. in an "interruptable thyristor, by contrast, the transconductivepath, once rendered conductive by the application of a pulse of therequired polarity to its control electrode, may subsequently be cut off(or interrupted") by the application, to the same control electrode, ofa pulse of the opposite polarity. In the particular arrangement shown inthe drawing, the interruptable" thyristor gate 6 will close to renderthe tube 5 inoperative during the time that the control electrode(designated 33) of the thyristor is negative, and will open to couplethe energizing voltage from the flash capacitor 2 to the tube 5 tocondition the latter for operation when the control electrode 33 ispositive.

The control electrode 33 of the thyristor gate 6 is coupled to theoutput of a bipolar gating device 16 (illustratively a Schmitt triggercircuit) whose output polarity is normally negative to maintain the gateclosed. The input of the Schmitt trigger 16 is arranged to be excited atdifferent times by the ignition transformer 10 and the output of thephotosensitive element 14. In particular, an adjustable tap point 34 onthe primary winding 9 of the ignition transformer 10 is coupled througha diode 11 and a resistor 12 to the input of the Schmitt trigger l6 andto the junction of the photosensitive element 14 and the integratingcapacitor 15. Electrical excitation for the integrating path includingthe photosensitive element l4 and the capacitor 15 is taken across theresistor 3, which generates an operating voltage only when the flashtube 5 is in its conductive state.

In operation, the trigger contacts 8 are initially open and the input ofthe Schmitt trigger circuit 16 is quiescent. The resulting steadynegative output of the Schmitt trigger l6 maintains a steady negativepotential on the control gate 33 of the thyristor gate 6. The gate 6 isthus closed and prevents operating voltage from the flash capacitor 2from exciting the main electrodes 21 and 22 of the flash tube 5.

When the trigger contacts 8 are momentarily closed, the primary winding9 of the transformer 10 is short-circuited through the storage capacitor7 which has been initially charged from the DC power source 1, like theflash capacitor 2. The capacitor 7 thereupon discharges through thewinding 9. The resulting negative voltage at the tap point 34 of thewinding 9 is coupled through the diode 11 and the resistor 12 tonegatively charge the capacitor and to excite the input of the Schmitttrigger 16, thereby reversing its output polarity. The resultantpositive impulse applied to the control electrode 33 of the gate 6 opensthe latter and couples operating voltage from the flash capacitor 2 tothe main electrodes 21 and 22 of the flash tube 5. The gate 6 willtemporarily remain in its conductive state because of the negativevoltage stored on the capacitor 15. Such stored voltage, which has amagnitude determined by the setting of the tap point 34, will behereafter referred to as the reference voltage.

As indicated before, the presence of the operating voltage across thetube 5 is not sufficient in and of itself to cause it to emit lightenergy. However, the capacitor discharge caused by closure of thecontacts 8 also causes the application of an ignition pulse to thetrigger electrode 13 of the flash tube 5 via the ignition transformer10, so that the combination of this ignition pulse and the now-presentoperating voltage across the main electrodes 21 and 22 drives the tube 5into its conductive state and causes it to emit light energy. Theconduction of the tube 5 also causes current flow through the resistor3, which generates excitation voltage for the integrating circuitconsisting of the photosensitive element 14 and the capacitor 15.

It is well known that a reversal of polarity at the output of theSchmitt trigger 16 may be effected upon the reversal of polarity at itsinput. Such input polarity reversal, in the illustrated configuration,may in turn occur when a positive voltage equal to the negativereference voltage on the capacitor 15 is applied thereto through thephotosensitive element 14. Such positive voltage is generated during theinterval that the flash tube 5 emits light, since the resistance of thephotosensitive element 14 is lowered sufficiently in a well-known mannerby the influence of the incoming light reflected from the object to bephotographed to charge the capacitor 15 in a positive sense from thevoltage source represented by the potential across the resistor 3. Whensuch flash duration has persisted for a duration long enough to allowthe buildup of positive charge on the capacitor 15 to yield a voltageequal to the stored reference value, the polarity at the output of theSchmitt trigger 16 will again be reversed to apply a negative disablingvoltage to the control electrode 33 of the gate 6. The resulting closureof the gate 6 stops the discharge of the flash capacitor 2 through thetube 5, so that the latter ceases to emit light energy. The resultingcutoff of the tube 5 disables the energizing voltage generated acrossthe resistor 3 for exciting photosensitive element 14. Since thepositive voltage built up on the capacitor 15 during the light flash isindicative of the 5 quantity of light integrated via the photosensitiveelement 14,

and since such voltage is also equal to the reference voltage determinedby the adjustment of the tap point 34 on the primary transformer 9, itis evident that the output quantity of light emitted from the tube 5before the flash is terminated can be adjusted to a desired value byappropriately setting the tap point 34 on the ignition transformer 10.Once this adjustment is made the duration of the flash interval of thetube 5 is automatically regulated to maintain the corresponding outputquantity of light energy.

It will be noted that unlike prior art light-integrating flash durationcontrols, the instant arrangement employs only so much of the energystored on the flash capacitor 2 that is necessary to produce usefulemitted light from the tube 5. In most instances, this will represent anincomplete discharge of the capacitor 2. Thus time necessary to restorefull voltage on the capacitor 2 from the power supply 1 in the intervalbetween successive flashes can be relatively short compared to the timenecessary to fully recharge the capacitor 2 from a completely dischargedstate which is manifested in prior art arrangements by the use of quenchtubes in parallel with the flash tube. Hence rapid and efiicientoperation of the flash unit is assured with the instant arrangement.

In the foregoing, the invention has been described in connection with apreferred arrangement thereof. However, since many other variations andmodifications will now become obvious to those skilled in the art, it isaccordingly desired that the breadth of the claims not be limited to thespecific disclosure herein contained.

lclaim:

1. In combination with a flash device having a transconductive path anda trigger electrode and operable to emit a flash of light upon theapplication thereto of operating voltage through the transconductivepath and the simultaneous application .of an igniting pulse to thetrigger electrode, an apparatus for automatically adjusting theoperating duration of the flash device to obtain a controllable quantityof light ener gy therefrom, which comprises:

normally disabled switching means interposed in the transconductive pathfor coupling the flash device to a source of operating voltage; v

detecting means for integrating a portion of the light energy containedin the emission from the flash device and for generating control signalthat varies in proportion to the amount of light integrated;

means for selectively applying an igniting pulse to the triggerelectrode; and

means coupled to the detecting means and to the applying means forenabling the switching means when an igniting pulse is applied to thetrigger electrode and for subsequently disabling the switching meanswhen the control signal has reached a predetermined value.

2. In an electronic timing apparatus wherein the duration of 'lightenergy emitted from a flash tube having a trigger electrode and a sourceof operating voltage is regulated to produce a predetermined quantity ofemitted light energy:

I detecting means for integrating a portion of the emitted light toproduce a control signal that varies in proportion to the amount oflight integrated;

a controllable gate serially connected with the main electrodes of theflash tube and the source of operating voltage and operative when opento apply operating voltage to the main electrodes, the gate having acontrol electrode responsive to signals of one polarity for opening thegate and responsive to signals of the opposite polarity for closing thegate;

means for applying an igniting pulse to the trigger electrode of theflash tube;

bipolar control means coupled to the applying means and to the detectingmeans for initially generating an output signal of the one polarity whenan igniting pulse is applied to the trigger electrode and forsubsequently terminating the last-mentioned signal and generating anoutput signal of the opposite polarity when the control signal hasreached a predetermined value; and

means for coupling the output signal of the bipolar control means to thecontrol electrode of the gate.

3. Apparatus as defined in claim 2 in; which the bipolar control meansis a Schmitt trigger circuit.

4. Apparatus as defined in claim 2, in which the controllable gate is aninterruptable thyristor having a transconductive path serially connectedwith the main electrodes of the flash tube, the transconductive pathbeing rendered conductive upon the application of a voltage of the onepolarity to its control electrode and rendered nonconductive upon theapplication of a voltage of the opposite polarity to its controlelectrode.

5. Apparatus as defined in claim 2, in which the detecting meanscomprises, in combination, a photosensitive element in operativeassociation with the light emitted from the flash tube;

a first capacitor; means for coupling the photosensitive element to thefirst capacitor and to the input of the bipolar means; and means forelectrically exciting the photosensitive element when the gate isopened.

6. Apparatus as defined in claim 5, in which the 'applying meanscomprises, in combination, an ignition transformer having a primarywinding coupled across the gate and the flash tube through a secondcapacitor;

a secondary winding coupled to the trigger electrode of the flash tube;and

means for momentarily short circuiting the primary winding and thesecond capacitor.

7. Apparatus as defined in claim 6 in which the primary winding has anadjustable tap, and in which the apparatus further comprises means forcoupling the adjustable tap to the input of the bipolar control meansand to the first capacitor for establishing a reference voltage at theinput of the bipolar control means.

1. In combination with a flash device having a transconductive path anda trigger electrode and operable to emit a flash of light upon theapplication thereto of operating voltage through the transconductivepath and the simultaneous application of an igniting pulse to thetrigger electrode, an apparatus for automatically adjusting theoperating duration of the flash device to obtain a controllable quantityof light energy therefrom, which comprises: normally disabled switchingmeans interposed in the transconductive path for coupling the flashdevice to a source of operating voltage; detecting means for integratinga portion of the light energy contained in the emission from the flashdevice and for generating control signal that varies in proportion tothe amount of light integrated; means for selectively applying anigniting pulse to the trigger electrode; and means coupled to thedetecting means and to the applying means for enabling the switchingmeans when an igniting pulse is applied to the trigger electrode and forsubsequently disabling the switching means when the control signal hasreached a predetermined value.
 2. In an electronic timing apparatuswherein the duration of light energy emitted from a flash tube having atrigger electrode and a source of operating voltage is regulated toproduce a predetermined quantity of emitted light energy: detectingmeans for integrating a portion of the emitted light to produce acontrol signal that varies in proportion to the amount of lightintegrated; a controllable gate serially connected with the mainelectrodes of the flash tube and the source of operating voltage andoperative when open to apply operating voltage to the main electrodes,the gate having a control electrode responsive to signals of onepolarity for opening the gate and responsive to signals of the oppositepolarity for closing the gate; means for applying an igniting pulse tothe trigger electrode of the flash tube; bipolar control means coupledto the applying means and to the detecting means for initiallygenerating an output signal of the one polarity when an igniting pulseis applied to the trigger electrode and for subsequently terminating thelast-mentioned signal and generating an output signal of the oppositepolarity when the control signal has reached a predetermined value; andmeans for coupling the output signal of the bipolar control means to thecontrol electrode of the gate.
 3. Apparatus as defined in claim 2 in;which the bipolar control means is a Schmitt trigger circuit. 4.Apparatus as defined in claim 2, in which the controllable gate is aninterruptable thyristor having a transconductive path serially connectedwith the main electrodes of the flash tube, the transconductive pathbeing rendered conductive upon the application of a voltage of the onepolarity to its control electrode and rendered nonconductive upon theapplication of a voltage of the opposite polarity to its controlelectrode.
 5. Apparatus as defined in claim 2, in which the detectingmeans comprises, in combination, a photosensitive element in operativeassociation with the light emitted from the flash tube; a firstcapacitor; means for coupling the photosensitive element to the firstcapacitor and to the input of the bipolar means; and means forelectrically exciting the photosensitive element when the gate isopened.
 6. Apparatus as defined in claim 5, in which the applying meanscomprises, in combination, an ignition transformer having a primarywinding coupled across the gate and the flash tube through a secondcapacitor; a secondary winding coupled to the trigger electrode of theflash tube; and means for momentarily short circuiting the primarywinding and the second capacitor.
 7. Apparatus as definEd in claim 6 inwhich the primary winding has an adjustable tap, and in which theapparatus further comprises means for coupling the adjustable tap to theinput of the bipolar control means and to the first capacitor forestablishing a reference voltage at the input of the bipolar controlmeans.